Glycogen Metabolism

Glycogen Metabolism Study Notes

Overview of Glycogen Metabolism

  • Glucose Storage

    • Glucose cannot be stored directly, as high concentrations disrupt osmotic balance, risking cell damage or death.

    • Instead, glucose is stored as glycogen, which is less osmotically active.

    • Glycogen is a highly branched polymer, enabling rapid breakdown into glucose molecules when energy is required.

    • Presence of glycogen observed in bacteria, archaea, protists, and animals.

    • Controlled release of glucose from glycogen maintains blood glucose levels between meals.

    • Glycogen serves as a quick energy source for sudden strenuous activity as it can be metabolized anaerobically (absence of O2).

    • Glycogen is present in granules within the cytoplasm of cells.

Structure of Glycogen

  • Composition of Glycogen

    • Individual glycogen molecules consist of approximately 12 layers of glucose molecules.

    • Glycogen molecules can grow to sizes up to 40nm and contain around 55,000 glucose residues.

    • The core of glycogen features a single protein called glycogenin.

  • Linkages

    • Glycogen comprises long chains of glucose residues linked via α1→4 glycosidic bonds.

    • Approximately every 12th residue is also linked by α1→6 bonds, forming branches of the chain.

Glycogen Degradation and Synthesis

  • Degradation Process

    • Glycogen is broken down in three steps yielding glucose-6-phosphate (G6P).

    • G6P can enter glycolysis or the citric acid cycle (CAC) for further energy production.

    • In the liver, G6P can be converted into free glucose.

    • G6P is also utilized in the pentose phosphate pathway (PPP) for NADPH production and ribose-5-phosphate formation.

  • Synthesis Process

    • Glycogen synthesis requires several steps and is stimulated when glucose concentration is elevated, enabling its storage.

Enzymes Involved in Glycogen Breakdown

  • Glycogen Phosphorylase

    • Catalyzes the phosphorolysis reaction, producing glucose-1-phosphate (G1P).

    • Preserves the α configuration of glucose during breakdown.

  • Debranching Enzyme

    • Responsible for relocating glucose residues to the main chain, ensuring complete glycogen breakdown.

    • Contains two functions: transferase, which moves glucose residues to the parent branch, and α-1,6 glucosidase, which hydrolyzes the last glucose on a branch to “delete” it.

  • Phosphoglucomutase (PGM)

    • Converts G1P to G6P via a two-step phosphorylation mechanism.

Phosphorylase Mechanism

  • Structure and Function

    • Glycogen phosphorylase is a homodimer with an active site that excludes water to prevent glucose release instead of G1P.

    • The glycogen binding site is located 30Å away from the active site, connected by a narrow crevice that allows for sequential phosphorolysis of residues (processive action).

  • Configuration Conservation

    • The α-1,4 linked glucose residues are released as α-D-glucose-6-phosphate, preserving the α configuration, indicating a carbocation intermediate mechanism during cleavage.

Debranching Enzyme

  • Functionality

    • Glycogen phosphorylase is limited to cleaving α1→4 bonds; it cannot act on short chains that do not reach the active site.

    • When four residues remain at the branch, the transferase function relocates three of these residues, while glucosidase hydrolyzes the remaining residue.

Phosphorylase Regulation

  • Allosteric and Covalent Control

    • Phosphorylase regulation includes allosteric effectors linked to the cell's energy state, alongside hormonal signaling involving glucagon, epinephrine, and insulin.

  • Isoforms of Phosphorylase

    • Two isozymes exist: liver (hormone-responsive) and muscle (energy state-responsive).

  • Active vs. Inactive Forms

    • Phosphorylase exists as an active phosphorylated a form and an inactive unphosphorylated b form.

    • The equilibrium exists between two states: active relaxed (R) and inactive tense (T).

    • The a form favors the R state, while the b form favors T state equilibrium.

Phosphorylase Behavior in Liver and Muscle

  • Liver Phosphorylase Dynamics

    • In the liver, the a form predominates to maintain blood glucose levels; glucose binding shifts the a form from R to T state, reducing activity.

    • At low glucose concentrations, the enzyme is in the active R state to facilitate glucose production for other tissues.

  • Muscle Phosphorylase Behavior

    • In muscle tissue, AMP stabilizes the R state, enhancing activity even in the b form, while ATP and G6P stabilize the T state, with ATP outcompeting AMP for binding.

Phosphorylation Mechanism

  • Phosphorylation Process

    • Phosphorylase kinase catalyzes the conversion from the inactive b form to the more active a form, triggered by glucagon or epinephrine responses.

    • The phosphorylation event moves the activation loop away from the active site, enhancing enzyme function.

  • Composition of Phosphorylase Kinase

    • Composed of subunits αβγδ:

    • α and β are phosphorylation targets; γ contains the active site; δ is calmodulin for Ca2+ binding.

    • Activation cooperatively occurs via calcium binding and concurrent phosphorylation by protein kinase A (PKA).

UDP-Glucose as an Activated Carrier

  • Importance

    • UDP-glucose acts as an activated carrier in the synthesis of glycogen, modifying glucose to enhance its reactivity for linkage.

Synthesis of UDP-Glucose

  • Conversion to UDP-Glucose

    • Synthesis occurs via the enzyme UDP-glucose pyrophosphorylase, producing PPi.

    • The synthesis reaction is reversible; however, it does not reverse in vivo due to thermodynamic factors.

Glycogen Synthase Functionality

  • Role of Glycogen Synthase

    • Glycogen synthase is the key regulatory enzyme, adding new glucosyl units to glycogen's non-reducing terminal residues, forming α-1,4-glycosidic linkages.

  • Initiation of Glycogen Synthesis

    • Glycogen synthase requires a primer (4+ residues) to begin synthesis, solved by the glycogenin dimer, which forms the primer and then allows glycogen synthase to elongate the chain.

    • Glycogenin attaches a glycosyl unit to a tyrosine residue in its active site, forming a chain through α1→4 linkages until approximately 20 residues long.

Branching Enzyme Functionality

  • Formation of Branches

    • Branching enzyme generates branches by cleaving α-1,4 linkages, transferring blocks of around 7 residues, and relinking them with α-1,6 linkages.

    • Requirements for the block: it must include the non-reducing terminus, and must be sourced from a chain containing at least 11 residues, with the new branch at least four residues away from existing branches.

Regulation of Glycogen Synthase

  • Forms of Glycogen Synthase

    • Exists in two forms: an active non-phosphorylated a form and an inactive phosphorylated b form.

  • Regulatory Phosphorylation

    • Glycogen synthase is inactivated via phosphorylation by glycogen synthase kinase (controlled by insulin) and PKA.

    • Glucose 6-phosphate serves as a powerful activator of glycogen synthase b, stabilizing its active state relative to the inactive state.

Reciprocal Regulation of Breakdown and Synthesis

  • Pathway Interactions

    • Glycogen synthesis is inhibited by hormones like glucagon and epinephrine, which promote breakdown.

    • PKA phosphorylates phosphorylase kinase, initiating breakdown; concurrently, glycogen synthase kinase and PKA phosphorylate glycogen synthase, reducing activity and inhibiting synthesis.

Regulation by Protein Phosphatase 1 (PP1)

  • Role of PP1

    • PP1 dephosphorylates various proteins to limit the rate of glycogen breakdown, affecting the following:

    • Inactivates phosphorylase a.

    • Inactivates phosphorylase kinase.

    • Activates glycogen synthase by converting glycogen synthase b to the active a form.

PP1 Activating Mechanisms

  • Regulatory Subunits

    • The catalytic subunit of PP1 binds to regulatory subunits such as GM in skeletal muscle and heart, and GL in the liver, serving as scaffolds to organize interactions.

  • cAMP Cascade Influence

    • The cAMP cascade activates PKA and reduces PP1 activity via two mechanisms, including GM phosphorylation leading to dissociation and phosphorylation of small proteins that inhibit PP1.

Insulin's Role in Glycogen Synthesis

  • Impact of Insulin

    • High blood glucose levels trigger insulin release, which inactivates glycogen synthase kinase through a signal transduction pathway, stimulating glycogen synthesis.

    • Insulin prevents glycogen synthase from being kept in an inactive phosphorylated state, allowing PP1 to activate it, restoring glycogen reserves.

    • Insulin enhances the number of glucose transporters in the membrane, facilitating glucose uptake.

Regulation of Blood Glucose by Liver Glycogen Metabolism

  • Feedback Mechanism

    • Active liver phosphorylase a decreases rapidly upon glucose infusion, shutting off glycogen breakdown at elevated glucose levels.

    • Glucose binding transitions phosphorylase a from active R to inactive T form, concurrently promoting the action of PP1 which converts phosphorylase to its b form.

    • Following a lag period, the activity of glycogen synthase a increases, leading to enhanced glycogen synthesis, aided by dephosphorylation by PP1, which activates glycogen synthase a.